1. Field of the Invention
The present invention relates to a method and apparatus for image forming, and more particularly to a method and apparatus for image forming capable of controlling toner-concentration accurately.
2. Discussion of the Background
An image forming apparatus that employs an electrophotographic method has been developed rapidly. Such an apparatus includes a printer, a copier, a facsimile machine, and a multi-function system, for example.
Recently, there is increasing demand that such an image forming apparatus have high stability and durability in addition to a high performance to obtain high quality images. Namely, it is requested that the image forming apparatus can maintain a constant quality of image forming that is less affected by environmental variation.
A background image forming apparatus commonly employs a two-component developer method using a two-component developer to visualize an image in an image forming operation because the two-component developer easily handles color images. The two-component developer (developer) includes non-magnetic toner and magnetic carrier.
In the two-component developer method, the background image forming apparatus holds the two-component developer on a developing sleeve which is a developer carrier. The background image forming apparatus forms a magnetic brush generated by magnetic poles provided in the developing sleeve. The two-component developer is conveyed to a developing region between the developing sleeve and a photoreceptor in accordance with a rotation of the developing sleeve. While the developer is conveyed to the developing region, a plurality of magnetic carriers in the developer are gathering together along a magnetic field generated by the magnetic poles to form the magnetic brush.
It is important to control a weight ratio of the toner and the carrier accurately to improve stability of the two-component developer. If toner-concentration is too high, scumming of the image may occur. As a result, resolution of a fine image may be decreased. Meanwhile, if toner-concentration is too low, another problems may occur. For example, low concentration may occur in a plain image area, or carrier adhesion may be generated.
To solve these problems, the toner-concentration of the developer needs to be adjusted to a necessary range by controlling the toner supply amount to the developer being used. Therefore, a sensor may be employed to detect the toner-concentration and to compare an output voltage of the sensor with a reference value of the toner-concentration. The toner supply amount is then determined based on the comparison result.
There are a variety of methods to detect toner-concentration. One method is to use a permeability sensor. A permeability of the developer changes when the toner-concentration of the developer is changed. The permeability sensor detects and compares a detected value with a reference value to determine if the toner supply amount needs to be adjusted.
Another method is to use a light sensor. In this method, a reference image pattern is formed on a photoreceptor, or an intermediate transfer belt initially. The light sensor detects light reflections from an image area having an actual image and a background area having no image. The toner-concentration of the developer is detected based on the detection result.
Further, the reference image pattern is transferred to paper from the photoreceptor or intermediate transfer belt during image forming process. The light sensor detects the light reflections from the image area and the background area on the paper. Then, a reference value Vref is controlled. However, in this method, toner is wasted because of the actual image forming on the photoreceptor, or the intermediate transfer belt, or during the transfer process to the paper.
In another background image forming apparatus, a controller detects toner-concentration of the developing unit and compares a detected value with a threshold value. The controller controls the toner-concentration of the developing unit by changing the threshold value by a predetermined value in accordance with a change of a linear velocity of a photoreceptor.
However, when the linear velocity of a photoreceptor is large, an output signal of the permeability sensor may be saturated. As a result, the toner-concentration can not be detected in the saturated region.